Deoxyribonucleic acid, often abbreviated to DNA, is found in the nucleus of the cells of almost all living organisms on earth. DNA contains the genetic instructions for making proteins and how an organism will develop, live and reproduce, and is often referred to the building block of life for organisms (reference). DNA is arranged in a spiralling double helix shape, similar to a twisted ladder, and contains thousands of repeating nucleotides, which are the structural components of DNA. Each nucleotide is comprised of a deoxyribose sugar molecule and a phosphate group, which create the ‘backbone’ of each DNA strand, and a nitrogenous base. There are four bases that can be found within DNA, which include adenine, thymine, cytosine and guanine. These bases form base pairs, where each base will only pair with its complementary base. This means that adenine will only pair with thymine, and cytosine will only pair with guanine. These base pairs make up the ‘rungs’ of the ladder in DNA’s twisted helix formation. The order of the bases within each DNA molecule is what is used as a code to synthesise proteins which determine each characteristic of every living organism. DNA molecules are bound to and coiled around proteins called histones which allow a large amount of DNA to be stored within each nucleus. These coiled histones form a coiled network named chromatin which, when a cell is about to divide, coils even tighter into structures called chromosomes. This experiment was designed to extract clumped strands of DNA from strawberry cells using a solution of salt, detergent, water and ethanol. The aim of the experiment was to investigate the strands of DNA extracted from the strawberries and analyse the effect that changing the amount of detergent in the solution had on the weight of DNA that was produced from the cells. Strawberries are particularly useful when investigating DNA because they are octoploid, which means that each cell contains 8 copies of each chromosome, resulting in a large quantity of DNA that is able to be extracted in the experiment (Washington). Different quantities of detergent were added to the solutions whilst the amount of strawberries, salt, ethanol and water were controlled in order to test the effect that changing the concentrations of the detergent would have on the amount of DNA produced. The hypothesis of this experiment was that if more detergent was added to the solution, more DNA would be extracted from the strawberry cells.
As demonstrated by graph 1 and table 1, there was a linear trend that increases the weight of DNA collected as the amount of detergent was increased. This linear increase of DNA extracted can be suggested to be attributed to the structure and properties of detergent. Detergent is created from molecules that contain charged hydrophilic, or water-loving, heads and non-charged hydrophobic, or water-hating, ends (Detergent structure). This structure allows the detergent molecules to act as an emulsifier for lipids, which means that it has the ability to suspend and disperse small droplets of lipids in a water-based solution by trapping them in bubbles called micelles (emulsifier definition). Micelles have a similar structure to a cell’s membrane, as the both have a hydrophilic head facing out and hydrophobic tails facing in in a spherical shape. Because lipids are non-polar and insoluble in water, they are able to be broken down by the non-polar ends of the detergent and then trapped within the micelle. In this experiment, the detergent is able to release DNA from the cells by causing cell lysis, or breaking down, of the lipids in the phospholipid bilayer and rupturing the nuclear membrane where the strands of DNA are stored within a cell (Murdoch). As DNA is insoluble to ethanol, the ethanol that is added to the solution causes the DNA to precipitate into a white stringy substance which is then able to be extracted from the solution and weighed. As the concentration of detergent was increased in the solution, it was likely able to break down the cellular and nuclear membranes of the strawberry cells more effectively as the detergent was able to come into contact with a higher number of cells, and therefore release more DNA strands than the solutions with lower concentrations of detergent.
As the amount of detergent reached 4mL, the amount of DNA produced from the strawberries stopped increasing in a linear trend and levelled out at 6g of DNA produced. This lack of increase with a higher concentration of detergent could suggest that sufficient detergent was added to the solutions in order to release all the viable DNA, and that adding more detergent to the solution would not have an impact as the membranes are already completely broken down. The data produced from this experiment is linear and consistent and is therefore relatively both reliable and accurate. The data collection strategy was also considerably reliable, as the experiment was conducted in the same environment and the same time, and the controlled variables of the amount of strawberries, water, ethanol and salt meant that the amount of detergent was the only variable being altered. However, a limitation of the data collection strategy was that the scale used in the experiment only displayed measurements for every tenth of a gram and was therefore not as accurate in the exact weights of the DNA produced. Another limitation was that whilst the strawberries were all measured to 10g, the lack of a more accurate scale with smaller increments could have resulted in slightly larger amounts of strawberries in one of the solutions which could potentially have produced more DNA due to having a larger quantity of cells. The experiment could be adjusted to provide more accurate and reliable results by using a scale with smaller increments of measurement, by performing multiple trials to get an average for each different amount of detergent, and by adding smaller increments of detergent to be able to accurately see the change of DNA produced in relation to the concentration of detergent.
An experiment was conducted to test the amount of DNA that could be extracted from strawberry cells when changing the concentrations of detergent in the solution that was mixed with the strawberries. The results of the experiment were that the amount of DNA increased as more detergent was added, before levelling off after 3mL of detergent was added. The amount of DNA increased as more detergent was added because the detergent breaks down the lipids in the cellular membranes of cells, allowing them to rupture and release the DNA stored within the nucleus. The levelling of the amount of DNA produced after 3mL of detergent was added could be attributed to the membranes being fully broken down, and could suggest that no more detergent was required to break down and separate the lipids, and that 3mL was the optimum amount of detergent for the amount of strawberries. The experiment produced consistent linear results and therefore can be considered as relatively valid data. The hypothesis that as more detergent is added, a higher quantity of DNA will be produced was supported by the linear trend of the data from this experiment.
- (Washington) www.gs.washington.edu/outreach/Dhillon_dnaprocedure.pdf
- (R2 value) My Accounting Course. (2019). What is R Squared (R2)? – Definition | Meaning | Example. [online] Available at: https://www.myaccountingcourse.com/accounting-dictionary/r-squared [Accessed 19 Aug. 2019].
- (detergent structure) https://www.sigmaaldrich.com/technical-documents/articles/biofiles/detergent-properties.html
- (emulsifier definition) https://www.rimpro-india.com/articles1/surfactants-as-detergents-and-emulsifiers.html
- (image 1) https://cdn.instructables.com/FKR/QF9W/GYN8W52O/FKRQF9WGYN8W52O.LARGE.jpg?auto=webp&&frame=1
- (cell lysis and breakdown) http://www.explorecuriocity.org/portals/2/themes/biotechnology/DNA-Extraction-Backgrounder.pdf
- (Alcohol) https://info.gbiosciences.com/blog/bid/156468/work-of-salt-isopropanol-and-ethanol-in-dna-extraction
- (chromosomes) https://www.nature.com/scitable/topicpage/chromosomes-14121320/